In the bicycle art the terms "gear" and "sprocket" or "sprocket wheel" are often used interchangeably. In the present case the word gear will be used consistently throughout, it being understood that this term as used is equivalent to a sprocket.
Bicycle gear shifts generally are well known in the art and are usually designed to provide three, five, or ten distinct speeds. Normally where the number of gear positions is five, a rear gear cluster comprised of five gears in coaxial side-by-side relationship is provided together with a derailleur device for shifting the bicycle chain position laterally so that it will engage a desired gear in the cluster depending upon the particular selected gear ratio. In the case of a ten speed bike, there is normally provided a front gear cluster comprised of two gears having different numbers of teeth for cooperation with a rear gear cluster comprised of five gears. In these instances, a front derailleur is necessary to effect shifting of the bicycle chain between the two front gears making up the front gear cluster.
In most of the prior art ten speed bicycle gear shifts, two levers are provided on the frame connected to cables passing respectively to the front and rear derailleurs for operating the same. Each of the cables is normally maintained in tension by a strong spring built into the respective derailleur. Shifting of gears is accomplished by moving one or both of the derailleurs by pulling on the respective cables with the levers against the spring bias. A friction device holds each lever in its moved position so that the bicycle chain can be properly positioned on the selected front and rear gears. A further movement of either or both levers between their extreme positions will again operate the derailleurs to shift to another gear ratio.
Some of the problems encountered with the foregoing arrangements are as follows. First, for most gear positions, both of the levers must be operated. Since these levers are located normally on the down tube or head tube of the bicycle frame, a bicycle rider must continue to steer the bicycle with one hand on the handlebars and use his other hand to successively operate the levers. Moreover, the spring bias and friction hold must be overcome to move the levers. Second, the shift lever stroke is determined by "feel" and guesswork. There is no positive positioning of the derailleur because it has a continuous motion that passes through all intermediate positions. Normally, the shifting is completed when the bicycle chain is caught up on the selected gear and the derailleur may not always be in a consistent position for that particular gear, again depending upon the physical manner in which the lever is operated. Third, to shift the gears on the conventional prior art gear shift bicycles in proper sequence requires a mental analysis on the part of the bicycle rider or a memorization of the proper operating sequences of the two levers. In this respect, there is no easy means for the bicycle rider to determine the particular gear position. Normally he must examine visually the particular gears in the cluster over which the bicycle chain is passing.
In addition to the foregoing, with conventional known gear shifting arrangements, the progression of gear ratio values through ten speeds is erratic; that is, the change in the gear ratio value from one shift to a next successive shift can vary from 5 per cent to 19 per cent. Moreover, the shifting of the front and rear derailleurs when needed is not a consequence of a single lever operation, but rather must be done consecutively resulting in unwanted intermediate gear settings.